1. Field of the Invention
The present invention generally relates to supervisory control of an optical amplifying repeater using erbium-doped fibers and, more particularly, relates to a structure of a modulator circuit which monitors the operating state of a repeater and superposes a response signal as to the monitored state on the main signal to thereby inform the terminal station of the operating state.
2. Description of the Related Art
An optical amplifier directly amplifying an optical signal as it is, without converting the optical signal into an electrical signal, is substantially bit rate-free and has such features that it facilitates construction of a large-capacity system and enables multiple channels to be amplified en bloc. Because of such features, intensive studies are being made in various research institutions on the optical amplifier as one of the key devices in the optical communication system for future. As one of the types of such optical amplifier, an optical fiber amplifier using an optical fiber, chiefly the core of which is doped with a rare earth element or ion (such as Er, Nd, and Yb), is receiving keen interest. The optical fiber amplifier has such excellent characteristics that it provides high gain, its gain is not dependent on polarization, it produces low noise, and it incurs little connection loss at its connection with an optical fiber as a transmission line.
There has been made an attempt to construct a long-distance optical communication system by disposing a plurality of optical fiber amplifiers of the described type in an optical transmission line at intervals of a predetermined distance so that an optical signal is directly amplified by optical amplifying repeaters constituted of the optical fiber amplifiers inserted in the optical transmission line. When constructing such an optical communication system, it is preferable that a terminal station connected with the optical transmission line monitors the operating state of each optical amplifying repeater and executes necessary control of each optical amplifying repeater. In response to a supervisory signal output from the terminal station, a response signal indicative of the operating state of the repeater is sent back superposed on the main signal to the terminal station. Then, since the response signal is superposed on the main signal, there arises the need for an optical amplifying repeater including a modulator circuit which is capable of keeping the index of modulation of the response signal relative to the main signal virtually constant.
Referring to FIG. 1, in which a block diagram of a prior art repeater circuit is shown, reference numeral 10 denotes a repeater circuit for the up transmission line and 10' denotes a repeater circuit for the down transmission line. Signal light formed of the main signal and a supervisory control signal superposed thereon transmitted over the up transmission line 11 is input to an erbium-doped fiber 1. Pumping light from a pumping laser diode 6 is reflected by a wavelength division multiplexer 2 and enters the erbium-doped fiber and, thereby, the signal light is amplified in the erbium-doped fiber 1. The thus amplified signal light is transmitted through the wavelength division multiplexer 2 and most of it is transmitted through a beam splitter 3 to be output to the optical transmission line 11. A portion of the signal light is branched by the beam splitter 3 and enters a photodiode 4 and, therein, the optical signal is converted into an electric signal. The electric signal from the photodiode 4 is input to an optical output stabilizer circuit 5 and also input to a bandpass filter 8 whose center frequency is fl.
Only a subcarrier at a frequency fl (for example 10 MHz) modulated by the supervisory control signal is extracted through the bandpass filter 8 and input to a monitor and control circuit 9. In the monitor and control circuit 9, the subcarrier is demodulated and, thereby, the supervisory control signal transmitted from the terminal station, not shown, is reproduced. According to the supervisory control signal, input and output levels of the optical amplifying repeater are monitored, the driving current of the pumping laser diode 6 is monitored, the temperature of the repeater is monitored, the currently used pumping laser diode 6 is switched to a backup, and optical loopback is performed.
Further, the monitor and control circuit 9, in response to the supervisory control signal, generates a response signal for example at a frequency of 50 b/s and in an RZ format as shown in FIG. 2A. Further, it modulates, with the response signal, a sinusoidal signal output from an oscillator 12 generating the sinusoidal signal for example at a frequency of 10 KHz as shown in FIG. 2B, and inputs the modulated signal to a variable gain amplifier 7' through a monitor and control circuit 9' for the down transmission line. The variable gain amplifier 7' amplifies the input signal with a preset gain (G.sub.2) and outputs the amplified signal as an alternating current I.sub.AC for driving a pumping laser diode 6'.
Meanwhile, signal light input from the down optical transmission line 11' is supplied to an erbium-doped fiber 1' having optical amplifying function and this signal light is amplified by stimulated emission of radiation caused by the pumping from the pumping laser diode 6'. The thus amplified signal light is transmitted through a wavelength division multiplexer 2' and most of it is further transmitted through a beam splitter 3' to be output to the optical transmission line 11'. A portion of the signal light is branched by the beam splitter 3' and input to a photodiode 4' to be converted into an electric signal. The electric signal from the photodiode 4' is input to an optical output stabilizer circuit 5' and also input to a bandpass filter 8' whose center frequency is fl.
In the optical output stabilizer circuit 5', the average value of the input signal is obtained and this average value is compared with a preset reference value to obtain the difference therebetween, and a DC current ' I.sub.DC proportional to the voltage of the difference, for driving the pumping laser diode 6', is output. Further, an AC current I.sub.AC in accordance with the response signal is superposed on the DC current I.sub.DC and, with this current, the pumping laser diode 6' is driven. The pumping light output from the pumping laser diode 6' is input to the wavelength division multiplexer 2'.
Now, we suppose that for example the wavelengths of the signal light transmitted over the up and down optical transmission lines are each 1.55 micrometer, the wavelengths of the signal light from the pumping laser diodes 6 and 6' are each 1.48 micrometer, and both the wavelength division multiplexers 2 and 2' have characteristics to transmit light of a 1.55 micrometer band therethrough and reflect light of the wavelength of 1.48 micrometer. Then, the signal light input from the down optical transmission line 11' is transmitted through the wavelength division multiplexer 2' and input to the beam splitter 3' and, then, most of it is transmitted through the beam splitter 3' and a portion of it is reflected from the same. The pumping light from the pumping laser diode 6' is reflected by the wavelength division multiplexer 2' and introduced into the erbium-doped fiber 1'.
In the erbium-doped fiber 1', the signal light is optically amplified by the pumping light output from the pumping laser diode 6'. In this case, the main signal of the signal light input from the down transmission line 11' is for example a signal modulated in an NRZ format at a frequency of 2.5 Gb/s as shown in FIG. 2C, which, when the range of the time base is set to be on the order of milli second, is expressed as schematically shown in FIG. 2D.
The modulated wave (whose amplitude is denoted by B) by the above described response signal is superposed on the main signal (whose amplitude is denoted by A) as shown in FIG. 2E and the signal light is optically amplified with the index of modulation of the response signal component relative to the main signal set for example to 1%. Then, by means of the feedback loop through an optical output stabilizer circuit 5', the level of the output light of the optical amplifier using the erbium-doped fiber 1' is kept constant and, at the same time, the amplitude of the response signal superposed on the main signal is kept constant, and thus the response signal is returned, through the down transmission line 11', to the terminal station (not shown) which transmitted the supervisory control signal.
In the above described prior art repeater circuit, however, in order to maintain the output light level of the optical amplifying repeater to be constant, a portion of the output light is monitored, and the power of the output light of the pumping laser diode is controlled as well as the gain of the erbium-doped fiber is controlled such that the monitored output light level is held constant. Consequently, when the amplitude of the modulated wave by the response signal is held constant, the index of modulation of the response signal component relative to the main signal will vary with a change in the level of the input signal to the optical amplifying repeater, a change in the temperature thereof, and the like.
More specifically, when the amplitude B of the modulated wave by the response signal is held constant, the DC current component ID.sub.C will vary with a change in the level of the input signal to the optical amplifying repeater and, thereby, the index of modulation of the response signal component relative to the main signal will vary
Further, when the temperature is changed, the driving current (DC current component I.sub.DC) versus output characteristics of the pumping laser diode will vary and, hence, the index of modulation of the response signal component relative to the main signal will vary. When the index of modulation of the response signal component relative to the main signal varies so much as to exceed a predetermined limit, there arises a problem that the terminal station becomes unable to receive the response signal.